The Advanced Television Systems Committee (ATSC) standard for digital high definition television (HDTV) transmission uses a signal that is modulated as an eight (8) level vestigial sideband (VSB) symbol stream with a rate of 10.76 MHz. The signal is designated as ATSC 8-VSB. The ATSC standard defines a data segment as a length of data having eight hundred thirty two (832) symbols.
Each data segment begins with a fixed pattern comprising the four symbols “1001”. The fixed pattern of “1001” indicates the start of a new data segment. The “1001” symbols are the first four symbols of the 832 symbols in the data segment.
In the demodulation process, the fixed pattern “1001” is used by the receiver to determine the positional alignment of the data segment. Locating (or “recovering”) the positional alignment of the data segment is essential for data frame synchronization and for facilitating symbol timing recovery. Consequently, reliable and robust detection of the recurring four symbol “1001” pattern is essential to the operation of any receiver that receives signals that are broadcast according to the ATSC standard.
To enable robust data reception in the presence of noise, attenuation, and multipath reflections, a pilot carrier (also referred to as a “tone”) is added to the transmitted signal. The pilot carrier is located exactly at DC on the frequency spectrum. The pilot carrier is used in the receiver to precisely estimate the location of the received spectrum. The pilot carrier is also used in the receiver to synchronize a local oscillator in the receiver to the phase and frequency of the transmitter oscillator. The synchronization function is usually carried out in a circuit called the “carrier recovery unit.”
Synchronization algorithms are used to synchronize the receiver to the transmitter in three respects. The receiver is synchronized to the transmitter with respect to the instant of sampling and the sample rate. The receiver is also synchronized to the transmitter with respect to the alignment of data frame boundaries. The receiver is also synchronized to the transmitter with respect to the frequency and phase with which the received signal should be demodulated.
A typical carrier recovery unit uses a frequency phased lock loop (FFLP) to recover the pilot carrier. One of the major difficulties with this technique is that it uses non-linear circuits. Non-linear circuits have non-linear transfer functions that are difficult to analyze. In addition, the acquisition loop bandwidth in a frequency phased locked loop has to be narrow in order to obtain good tracking performance.
There is therefore a need in the art for a pilot carrier recovery circuit for single carrier signals that has a linear transfer function.
In order to obtain larger acquisition ranges, there is also a need in the art for a pilot carrier recovery unit for single carrier signals that has an acquisition bandwidth that is independent of tracking bandwidth.